This invention relates to a method and device for non-destructive inspection, by means of holographic interferometry, of details of machine units, mechanisms, and various materials. More particularly, the invention relates to a method/device, which makes it possible to considerably decrease the requirements for protecting against vibratory movements during measurements, additionally performing insitu measurements in real time of different objects under various weather conditions.
Optical holographic interferometry makes it possible to perform non-destructive inspections of blocks and units of machines and devices for eventual presence of internal defects, as well as measuring stresses of an object during the object""s work load and residual stresses caused by technological processes of welding, forging, soldering etc. These measurements are useful for such fields as offshore petroleum industry, shipbuilding industry, manufacturing industry, aircraft industry, and for all kinds of structures where loading stresses and residual stresses can result in failures.
The principle of non-destructive inspections of an area of an object by optical holographic interferometry can be described as follows: First, a hologram of the area which is to be investigated on the object is recorded and developed by a registering medium when the object is in an unloaded initial stage of stress (a description of recording holograms by a holographic interferometer is for instance given in the applicants""Norwegian application no. 19995311, which is incorporated by reference). Then one slightly reforms the investigation area of the object by applying some types of loading; for example stretching, compression, bending, twisting, heating, and combinations of one or several of these, etc. The loading is performed in such a way that the stress will be concentrated in the region where the eventual defect is located. Thereafter, the registering medium containing the developed image of the hologram and the object are simultaneously illuminated by coherent light. As a result, two light waves scattered by the investigation area before and after loading will simultaneously emerge behind the registering medium. These light waves will interfere and form an interferogram which contains a set of fringes. When the interferogram is observed, by example, the naked eye or an objective and a monitor through the registering medium, it is seen as a set of fringes which cover the investigation area during loading. In addition, a presence of areas with abnormal fringe behaviour corresponds to presence of defects in the object. With this technique, it is thus possible to reveal various types of defects such as cracks, conglutinations, incomplete fusion, voids, cavities, pores, etc.
It is also possible to employ this technique to determine load stresses and residual stresses in an investigation area of the object. For example, in the case of determining residual stresses, a hologram of the area in its initial state is registered and developed before a release of the residual stresses are performed at a small zone of the investigation area. The release of residual stresses leads to deformations in the vicinity of the zone of released stresses under the action of the residual stresses, and the value of the normal component of the surface displacement at the edge of this zone is subject of measurements since it is directly proportional to the residual stress value. Then the registering medium with the developed holographic image of the investigation area in it""s initial state and the investigation area with the zone of released residual stresses, are simultaneously illuminated with coherent light. The illumination forms an interferogram from which one first determines the normal components of the surface displacements at the edge zone of released stresses. Finally, the measurements are employed to calculate the magnitude of the residual stresses. This procedure is thoroughly presented in the applicants Norwegian application no. 19995312, which is incorporated here as a reference.
The presently known methods of holographic interferometry have some drawbacks which have hindered a wide use of this technique:
1) It is necessary to rigidly protect the processes of registration of holograms and formation of interferograms from vibrations. That is, to ensure conditions which excludes any relative movement of the investigation area, laser, the elements of the interferometer, and the registering medium in regard to each other. This can, for instance, be connected to the following; The spatial carrier frequency is typically in the order of 1000-2000 mmxe2x88x921, thus a relative displacement of one of the above mentioned components by as little as 0,5-1 xcexcm will result in a crabbing of the interference pattern of the hologram and render it""s registration impossible.
2) In the cases where the registering media are films of amorphous molecular semiconductors (AMS-films), one should perform the registration and development of holograms in comfortable conditions such as an in-house location in order to satisfy the sensitivity toward humidity and temperature of the registering medium. AMS-films are subject to an electrostatic charging by corona discharges prior to registration of holograms. This is impossible in a high humidity environment or at temperatures below 0xc2x0 C. Also, high quality registrations and developments of holograms becomes impossible in high humidities and low temperatures due to a surface relaxation of the variable component of the latent electrostatic image. Also, achieving an optimum heating rate of the AMS-film during development becomes impossible in such conditions.
3) The exceptional sensitivity of the holographic interferometry technique towards relative displacements of the elements of the device with respect to each other, was partially overcome by creation of devices which rigidly fastened all above-mentioned elements and the object which is to be investigated on a common base. However, a such devices appear to be unsuitable for objects which are subject to vibrations. This is partially due to the relative large number of components which have to be rigidly fastened relative to each other, and partially due to the shape and size of the object. Some objects are simply too large and heavy to be placed on a common base. Besides, these devices do not permit inspections of objects in situ.
The main object of the invention is to provide a device and method for non-destructive inspections of objects by means of the optical holographic interferometry technique in real-time scale and which overcomes the above mentioned drawbacks.
Another object of the invention is to provide a device and method for performing non-destructive inspections of objects by means of real-time optical holographic interferometry which makes it possible to perform the loading and illumination of the investigation area of an object with coherent light in situ, while the registration, development of holograms and formation of interferograms takes place in another location.
A further object of the invention is to provide a device and method for performing non-destructive inspections of objects by real-time optical holographic interferometry which considerably reduces the requirements for vibration protection during registration and development of holograms and formation of interferograms.
It is also an object of the invention to provide a device and method for performing non-destructive inspections of objects with the use of real-time optical holographic interferometry in situ.